This invention relates generally to electrostatography, and more particularly, concerns a method and apparatus for forming high quality images in an electrostatic printing machine.
Generally, processes for electrostatographic copying and printing are initiated by selectively charging and/or discharging a charge receptive imaging member in accordance with an original input document or an imaging signal, generating an electrostatic latent image on the imaging member. This latent image is subsequently developed into a visible image by a process in which charged developing material is deposited onto the surface of the latent image bearing member, wherein charged solids in the developing material adhere to image areas of the latent image. The developing material typically comprises carrier granules having charged marking or toner solids adhering triboelectrically thereto, wherein the toner solids are electrostatically attracted from the carrier granules to the latent image areas to create a powder toner image on the imaging member.
Alternatively, the developing material may comprise a liquid developing material comprising a carrier liquid having pigmented marking solids (or so-called toner solids) and charge director materials dispersed and/or dissolved therein (so-called carrier liquid), wherein the liquid developing material is applied to the latent image bearing imaging member with the marking solids being attracted to the image areas of the latent image to form a developed liquid toner image. Regardless of the type of developing material employed, the charged toner or marking solids of the developing material are electrostatically attracted to the latent image to form a visible developed image corresponding to the latent image on the imaging member.
The developed image is subsequently transferred, either directly or indirectly, from the imaging member to a copy substrate, such as paper or the like, to produce a "hard copy" output document. In a final step, the imaging member is cleaned to remove any charge and/or residual developing material therefrom in preparation for a subsequent image forming cycle.
The above-described electrostatographic printing process is well known and has been implemented in various forms in the marketplace to facilitate, for example, so-called light lens copying of an original document, as well as for printing of electronically generated or digitally stored images where the electrostatic latent image is formed via a modulated laser beam. Analogous processes also exist in other electrostatic printing applications such as, for example, ionographic printing and reproduction where charge is deposited in image-wise configuration on a dielectric charge retentive surface. It will be understood that the instant invention applies to all various types of electrostatic printing systems and is not intended to be limited by the manner in which the image is formed on the imaging member or the nature of the latent image bearing member itself.
As described hereinabove, the typical electrostatographic printing process includes a conventional development step whereby developing material including charged marking or toner solids is physically transported into contact with the imaging member so as to selectively adhere to the latent image areas thereon in an image-wise configuration. Development of the latent image is usually accomplished by electrical attraction of charged toner or marking solids to the image areas of the latent image. The development process is most effectively accomplished when the solids carry electrical charges opposite in polarity to the latent image charges, with the amount of toner or marking solids attracted to the latent image being proportional to the electrical field associated with the image areas. Some electrostatic imaging systems operate in a manner wherein the latent image includes charged image areas for attracting developer material (so-called charged area development (CAD), or "write white" systems), while other printing processes operate in a manner such that discharged areas attract developing material (so-called discharged area development (DAD), or "write black" systems).
The following disclosures may be relevant to some aspects of the present invention. U.S. Pat. No. 5,387,760 discloses a wet development apparatus for use in a recording machine to develop a latent image on a uniformly charged imaging carrier member toner image. The apparatus includes a development roller disposed in contact with or near the electrostatic latent image carrier and an application head for applying a uniform layer of the wet developer to the roller.
U.S. Pat. No. 5,436,706 discloses an liquid immersion development (LID) machine including a first member having a uniformly charged first surface having formed thereon a latent electrostatic image, wherein the latent electrostatic image includes image regions at a first voltage and background regions at a second voltage. A second member charged to a third voltage intermediate the first and second voltages is also provided, having a second surface adapted for resilient engagement with the first surface. A third member is provided, adapted for resilient contact with the second surface in a transfer region. The liquid immersion development (LID) machine also includes an apparatus for supplying liquid toner to the transfer region thereby forming on the second surface a thin layer of liquid toner containing a relatively high concentration of charged toner solids, as well as an apparatus for developing the latent image by selective transferring portions of the layer of liquid toner from the second surface to the first surface.
U.S. Pat. No. 5,619,313 discloses a method and apparatus for simultaneously developing and transferring a liquid toner image. The method includes the steps of moving a photoreceptor including a charge bearing surface having a first electrical potential, applying a uniform layer of charge having a second electrical potential onto the charge bearing surface, and image-wise dissipating charge from selected portions on the uniformly charged charge bearing surface to form a latent image electrostatically, such that the charge-dissipated portions of the charge bearing surface have the first electrical potential of the charge bearing surface. The method also includes the steps of moving an intermediate transfer member biased to a third electrical potential that lies between said first and said second potentials, into a nip forming relationship with the moving imaging member to form a process nip. The method further includes the step of introducing charged liquid toner having a fourth electrical potential into the process nip, such that the liquid toner sandwiched within the nip simultaneously develops image portions of the latent image onto the intermediate transfer member, and background portions of the latent image onto the charge bearing surface of the photoreceptor.
Image quality is a concern with all electrostatographic printing applications or toner image forming methods including the conventional exemplary methods discussed above. In such methods, image quality in electrostatographic printing applications may vary significantly and unacceptably due to numerous conditions affecting latent image formation as well as development, among various other factors. In particular, image development can be effected by charge levels, both in the latent image, as well as in the developing material. For example, when the charge on dry toner solids becomes significantly depleted, binding forces with the carrier also become depleted, causing an undesirable increase in image development, which, in turn, causes the development of the latent image to spread beyond the area defined thereby. The unacceptable result is often unwanted toner solids in background or non-image areas.
Costly and high precision charging and development devices are often not desirable solutions to unacceptable image quality. There is therefore, for example, an ongoing need for a method and apparatus for forming high quality toner images in a printing machine that do not result in images having poor quality backgrounds.